1. Field of the Invention
This invention relates to an injection moulding method for a material, such as metal, ceramics or plastic, and a mould clamping apparatus for an injection moulding machine.
2. Description of the Prior Art
It is known that an injection moulding machine generally comprises an injection apparatus for melting a material supplied thereto and then injecting the molten material into the cavity of moulds and a mould clamping apparatus for applying predetermined clamping force to the moulds. The molten material is injected into the cavity by moving an injection piston. In an actual injection moulding process, as high quality of a moulding article is demanded, it is also demanded to increase the necessary injection pressure per unit projection area of the moulding article and the mould clamping force for clamping the moulds against the injection pressure (to be specific, against the pressure of the molten material in the cavity that is proportional to the injection pressure). The injection pressure is normally set at 500.about.5000 Kg/cm.sup.2. But, the pressure of the molten material in the cavity is reduced to 30.about.70% due to the viscosity of the molten material and the resistance in the liquid passage behind a nozzle and thus becomes 250.about.2000 Kg/cm.sup.2. Large injection pressure is pre-set for a molten material with a high viscosity, or a large liquid passage, which greatly reduces the pressure of the molten material. The mould clamping force should be at least 250.about.2000 Kg/cm.sup.2 per unit projection area of a moulding article. Therefore, even for a moulding article with 5 cm.sup.2 -projection area, the mould clamping force of about 10 tons may be required depending on a demanded quality or a material used.
According to a conventional injection moulding method, a movable platen is moved toward a stationary platen to press the movable mould against the stationary mould and thus close these moulds. Then, mould clamping force is applied to the moulds to prevent the moulds from being opened by the injection pressure of a molten material that is to be injected into the cavity. The injection piston is moved to inject the molten material into the cavity. The mould clamping force is released to separate the movable platen from the stationary platen, thereby opening the moulds. Then, the moulding article is ejected from the cavity. As explained above, the conventional injection moulding method involve a step of applying the mould clamping force to the moulds before injecting the molten material into the cavity.
There are three well-known types of mould clamping apparatuses for an injection moulding machine, namely, a straight hydraulic mould clamping apparatus, a toggle type mould clamping apparatus and a direct clamp type mould clamping apparatus.
In the straight hydraulic mould clamping apparatus, a movable platen is directly coupled to the distal end of the piston inside the hydraulic mould clamping cylinder, and pressurized oil is supplied to the hydraulic cylinder to produce the mould clamping force. To quickly open or close the moulds, a mould opening/closing cylinder with a small diameter is provided with a mould clamping hydraulic cylinder having a large diameter in such a manner that a mould opening/closing piston is moved inside this cylinder.
The mould clamping force generated in the straight hydraulic mould clamping apparatus is proportional to the cross-sectional area of the hydraulic cylinder, which normally should have a large diameter, thus enlarging the mould clamping apparatus.
In contrast, the toggle type mould clamping apparatus increases the mould clamping force acting on the piston, using a toggle mechanism. This system can easily provide large mould clamping force as compared with the diameter of the hydraulic cylinder. But, the provision of the toggle mechanism complicates the mould clamping apparatus and enlarges the apparatus.
In the direct clamp type mould clamping apparatus, a mould clamping cylinder is mounted in the stationary platen, to which pressurized oil is supplied to apply tensile force to tie bars. This tensile force directly pulls the tie bars, thus clamping the moulds. In other words, the direct clamp type mould clamping apparatus moves the piston in the hydraulic cylinder to close the moulds and applies a specific magnitude of mould clamping force to the moulds. And, the movable platen is clamped by a locking mechanism such as a half nut or a lock plate. Then, the pressurized oil is supplied to the mould clamping cylinder so as to pull the tie bars which serve as the mould clamping piston, thus further producing the mould clamping force. In this apparatus, the overall length of the mould clamping apparatus can be shortened, but it is difficult to attain large mould clamping force.
However, all of the three types of mould clamping apparatuses have the same basic structure wherein the pressurized oil is supplied to the hydraulic cylinder to move the piston, which pushes the movable platen coupled to the piston to produce the mould clamping force. In these types of apparatuses, after the mould clamping force is applied onto the moulds, a molten material is injected into the cavity. However, the mould clamping force generated in the conventional apparatuses is directly proportional to the cross-sectional area of the hydraulic cylinder, so that producing desired mould clamping force inevitably requires a hydraulic cylinder and a piston both with large cross-sectional area. Thus, it is difficult to make the mould clamping apparatus compact without complicating its structure. This drawback hinders the injection moulding machine from becoming compact.
As is clear from the above, the conventional mould clamping apparatus generally involves a hydraulic driving system which uses pressurized oil to move the piston. But, there is also another type of mould clamping apparatus using a motor-driven system in which a ball screw is formed in the periphery of the piston and the driving force of a servo motor is transmitted to the screw to move the piston. This mould clamping apparatus can quickly move the piston, thus shortening the time to open or close the moulds. But, in this system, attainment of desired mould clamping force necessitates the piston to have a larger diameter to increase the strength of the piston. What is more, to provide large mould clamping force, it is necessary to use a servo motor with large cross-sectional area. This makes it difficult to provide a smaller mould clamping apparatus and does not therefore contribute to making the injection moulding machine smaller.